The xylenes, para-xylene, meta-xylene and ortho-xylene, are important intermediates which find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production. The distribution of xylene isomers from catalytic reforming and other sources generally does not match that of the sought isomers for chemical intermediates and thus the producer converts the feedstocks to generate more of the sought isomers.
The production of xylenes is practiced commercially in large-scale facilities and is highly competitive. Concerns exist not only about the effective conversion of feedstock through one or more of isomerization, transalkylation and disproportionation to product xylenes, but also other competitive aspects with respect to such facilities including capital costs and energy costs. A prior art aromatics complex flow scheme has been disclosed by Meyers in part 2 of the Handbook of Petroleum Refining Processes, Second Edition, 1997, published by McGraw-Hill.
Various sources have been proposed for monocyclic aromatics as a feed to a xylene production facility. The most prevalent are the catalytic reforming of naphtha fractions and pyrolysis followed by hydrotreating of naphtha fractions. These processes typically produce a wide spectrum of chemical compounds including not only the sought monocyclic aromatics but also polycyclic aromatics and olefins. Polycyclic aromatics and olefins are typically undesirable impurities in xylene production facilities. They can have a negative impact on the product quality and the efficiency of the processes such as by requiring additional process steps, reducing catalyst life, decreasing stability of the product, and causing undesirable product color. Polycyclic aromatics are typically removed by distillation from the desired monocyclic aromatics. These removed polycyclic aromatics are then disposed of in any suitable manner, usually as a fuel, and thus have lesser value. It is also known that the polycyclic aromatics can be converted to useful monocyclic aromatics such as toluene, xylenes and C9+ monocyclic aromatics
The quality of feed streams to the various process units within a xylene production facility is also specified to ensure proper performance. For example, the olefin content of streams fed to some process units of xylene production facilities, including the transalkylation unit, is limited. Thus, olefin are recognized as a contaminant in transalkylation feed and the conventional practice is to reduce the olefin content to an acceptable level (feed specification limit) using various olefin removal processes such as hydrotreating, hydrogenation, and treating with clay and/or molecular sieves. Olefins are for example commonly removed from the xylene production facility feedstock and/or intermediate streams at various locations within the facility by clay treating. In clay treaters, olefins are converted to oligomers which can cause fouling of the clay. The cost to operate clay treaters, including reloading them with fresh clay and disposal of the organic contaminated spent clay, can be a significant financial burden on the commercial-scale producer of xylenes. Moreover, clay treaters can result in alkylation of an olefin to an aromatic ring. Hence, the effluent from a clay treater can contain aromatic rings having C2+ substituents such as ethylbenzene, propylbenzene, and methylethylbenzene. Thus the value of the aromatic feedstock for the production of benzene, toluene and xylene is reduced.